A centrifugal separator of the kind referred is shown in WO 88/102664. Because the inlet opening of the outlet device in this centrifugal separator during operation is located only partly radially outside the free a liquid surface, a liquid of a high viscosity can be separated in and discharged out of such a centrifugal separator. However, the flow conditions in the flow channel near the inlet opening are such that a part of a liquid of high viscosity flowing into the flow channel turns and flows out again into the outlet chamber through the part of the inlet: opening located radially inside the free liquid surface. This results in a low outlet pressure, a great risk of air admixture and an increased risk of instability.
If one tries to reduce the return flow of separated liquid from the flow channel out through the inlet opening to the outlet chamber by diminishing the radius of the free liquid surface a wave is created in front of the inlet opening, the energy consumption increases to a high degree and the outlet pressure becomes low.